The collective term hydroxystilbenes encompasses a large group of compounds which can be obtained as natural constituents of herbal drugs or by chemical synthesis and some of which have pharmacological activity.
Thus, for example, Aggarwal et al. describe in a review article (Anticancer Research, 24, 2004) resveratrol and structurally similar compounds and the natural sources thereof.
Because of the potential pharmacological use of naturally occurring hydroxystilbenes and hydroxystilbene mixtures, there is a need to develop processes making it possible to isolate desired hydroxystilbenes reliably and reproducibly on the industrial scale and moreover in adequate purity and yield.
FR 2 835 185 describes a complex rhubarb extract obtainable from rhizomes of Rheum rhaponticum, which is said to be characterized in that it comprises at least 50% hydroxystilbenes, with at least 50% of these hydroxystilbenes consisting of rhaponticin, deoxyrhaponticin, astrangin and piceatannol. A preferred extract comprises 15-50% by weight rhaponticin, 10-35% by weight deoxyrhaponticin, 5-10% by weight astrangin and 0.1-3% by weight piceatannol. This extract is, as illustrated in the examples, prepared by hydroalcoholic extraction of rhizomes of Rheum rhaponticum. The total content of rhaponticin and deoxyrhaponticin which can be obtained thereby is only 76% by weight. The content of astrangin comprises 11% by weight, the content of piceatannol comprises 3% by weight, and the content of anthracenosides comprises 0.5% by weight. In addition thereto, this extract appears to comprise about 10% by weight further undefined constituents. It is additionally asserted in FR 2 835 185 that the specific extract therein has, as a result of alleged synergistic effects of the various ingredients of the extract, biological properties which are considerably superior to the effect of the individual hydroxystilbenes, especially those effects which the ingredients described therein are said to have individually. The extract described therein is alleged to have antioxidant, antitumor, antiinflammatory and estrogenic properties. However, in fact, FR 2 835 185 does not provide a verifiable technical teaching for the asserted pharmacological usability, to say nothing of the asserted synergistic effect of the complex drug extract described therein. The experimental section describes merely individual formulation examples of capsules, tablets or creams. In particular, experimental data proving the alleged usability for the treatment of disorders connected with free radicals, such as, for example, accelerated aging, cancer, arteriosclerosis, wrinkles, inflammatory phenomena and the like, are completely lacking. The asserted suitability of a combination of the rhubarb extract described therein with a hop extract rich in prenyl flavonoids for the treatment of diseases standing with free radicals and/or for the treatment of hormonal imbalance such as amenorrhea, menopause, hot flushes etc., is not proved by any data either. It is moreover entirely unclear which of the components actually present in the extract described therein (rhaponticin, deoxyrhaponticin, astrangin, piceatannol, anthracenosides, and the unanalyzed constituents present in a content of 10%) contribute to the asserted pharmacological activity or, where appropriate, are in fact absolutely necessary for the asserted synergism. The actual disclosure of FR 2 835 185 should therefore be restricted to the preparation of a specific, complex rhubarb extract by hydroalcoholic extraction of rhizomes of Rheum rhaponticum and the preparation of specific hydroxystilbene derivatives, and the production of various pharmaceutical formulations.
Numerous further publications (cf., for example, Babu et al., Bioorg. Med. Chem. Lett. 14 (2004), 3841-3845; Matsuda et al., Bioorg. Med. Chem. 9 (2001), 41-50) propose the use of methanol as extractant for extracting the constituents of Rheum rhizoma. 
EP-A-1 140 097 describes the isolation of trihydroxystilbene compounds from plant material by extraction with aqueous solvent, followed by a specific combination of chromatographic steps. The aqueous solvent is in particular an alcohol-water mixture comprising 75% alcohol. Pure water is not used as extractant. Typical extracted plants are Vitis vinifera and Polygonum cuspidatum. 
However, the extracts obtained according to the prior art have very complex compositions, have only limited suitability as such for medical use, and require further purification steps.
There is thus a need for an improved process for producing hydroxystilbene-containing drug extracts.